Abstract Current diagnosis, prognosis, and management of multiple sclerosis and related neurological disorders primarily rely on magnetic resonance imaging (MRI), which is widely used in selecting patients for immune- modulatory treatment, monitoring disease activity, and predicting treatment response. However, MRI is limited to be a measure of overall changes in tissue water content and reflects only macroscopic tissue injuries, which may be caused by a combination of pathological activities ranging from edema and inflammation to demyelination and axonal loss as observed in MS. Without any specificity for myelination, the use of MRI as a primary measure of disease activity is found to poorly correlate with clinical outcomes in MS. This long- standing clinico-radiological paradox in MS is considered as a missing link to finding a cure for MS as it hampers efficacy evaluation of putative MS therapies, particularly myelin-repair therapies that are designed to promote long-term functional restoration. To overcome this clinico-radiological paradox in MS, we hypothesize that positron emission tomography (PET) imaging, when used in combination with myelin-specific radiotracers, will be able to directly detect and quantify changes of myelin distribution in the brain and spinal cord and that measurement will correlate with clinical outcomes. To date, we have identified two radiotracers for PET imaging that complement each other in detection and quantification demyelination and remyelination: one is myelin-targeted [11C]MeDAS that is designed to monitor global myelin distribution and the other is potassium- channel targeted [18F]3-F-4-AP that is designed to monitor exposure of axons after myelin damage. Given each radiotracer alone will not be able to provide ubiquitous information, we plan to combine the two radiotracers and determine their synergistic utility for accurate detection and quantification of demyelination and remyelination in different animal models of MS. In this project, the following specific aims will be addressed: Aim 1. Conduct sequential microPET imaging using [11C]MeDAS followed by [18F]3-F-4-AP in a LPC rat model and correlate the imaging results with demyelination and remyelination; Aim 2. Conduct sequential microPET imaging using [11C]MeDAS followed by [18F]3-F-4-AP in an EAE rat model. After immunization, the same animals will be scanned at various time points to determine the pharmacokinetics of each tracer in the spinal cord. Aim 3. Determine the time courses of demyelination and remyelination through cross-reference of the longitudinal MeDAS-PET and [18F]3-F-4-AP-PET scans in the same animals. Completion of this project will deepen our understanding of the disease mechanisms by accurately monitoring the time course of demyelination and/or remyelination in vivo and develop parametrics for direct correlation with functional recovery and efficacy evaluation of new drug discovery and development.